1. Field of the Invention
The invention relates to a transmission belt that includes an annular ring and a plurality of plate-shaped elements, and that is wound on two rotational members to transmit power between the two rotational members.
2. Description of the Related Art
A stepped transmission and a continuously variable transmission are generally used when power is transmitted between two rotational members. As the continuously variable transmission, a belt continuously variable transmission and a toroidal continuously variable transmission are available. In the belt continuously variable transmission, a transmission belt and two pulleys, that is, a drive pulley and a driven pulley are used to continuously change a gear ratio. As a transmission belt used in the belt continuously variable transmission, an endless annular belt is available. For example, the endless annular belt is formed by stacking a plurality of plates called elements or blocks in a thickness direction of the plate, annularly arranging the plates, and annularly connecting the plates using an annular metal band called a ring or a hoop.
When the transmission belt is wound on the two pulleys, that is, the drive pulley and the driven pulley, and the drive pulley is driven, a compression force in a direction in which the elements are stacked, that is, in a thickness direction of the element is applied to the elements from the drive pulley, according to a frictional force in an area where the elements and the drive pulley contact each other, and torque of the drive pulley. The compression force transmitted to the elements that are in contact with the drive pulley is transmitted to the elements that are in contact with the driven pulley, via the elements that are not wound on the pulleys, and are arranged in a straight line. When the compression force is transmitted to the elements that are in contact with the driven pulley, torque is generated to rotate the driven pulley, according to a frictional force in an area where the elements and the driven pulley contact each other, and the transmitted compression force. Thus, power is transmitted between the drive pulley and the driven pulley through the transmission belt.
Japanese Patent Application Publication No. 55-100443 (JP-A-55-100443) describes an example of the transmission belt and an example of the element used to form the transmission belt. The drive belt (transmission belt) described in the publication No. 55-100443 is wound on a V-shaped pulley, and has a substantially trapezoidal cross section. The transmission belt includes at least one metal circulation carrier (ring), and a plurality of lateral members (elements) slidably attached to the ring in a manner such that side surfaces of top portions in a plate-thickness direction contact each other. To make the transmission belt bendable, an inclined surface is formed in each element so that the plate thickness decreases toward an inside in a direction of a radius of the transmission belt (i.e., toward a position below a neutral line of the belt). Thus, the elements contact each other at inclination lines (rocking edges). The inclination line (rocking edge) is positioned at a border between the inclined surface and a portion of the side surface of the top portion, which is parallel to the opposite side surface of the top portion.
Japanese Patent Application Publication No. 4-83940 (JP-A-4-83940) describes a transmission belt that includes a stacking belt (ring), and a plurality of V-shaped blocks (elements). Each of the elements has a belt fitting groove in which the stacking belt is fitted. The elements are arranged in a direction in which the transmission belt rotates, and connected. A rocking edge is formed in each of the elements. When the transmission belt is bent, the elements adjacent to each other contact each other at the rocking edges that are positioned on a pitch circle. The elements include at least two types of elements that are different from each other in a distance from the rocking edge to a contact surface of the belt fitting groove, which contacts an innermost peripheral surface of the ring.
When the transmission belt described in each of the above-described publications is wound on the two pulleys, belt curve portions and belt straight portions are formed in the transmission belt. In each belt curve portion, the elements are wound on the pulley, and arranged in an arc. In each belt straight portion, the elements are not wound on the pulley, and arranged in a straight line. For example, in the publication No. 55-100443, the inclined surface is formed in each element so that the lower portion of the element is thin. Therefore, when the transmission belt is wound on the pulleys, the elements in each belt curve portion are arranged in a fan shape with respect to a center of the pulley. Thus, the elements in each belt curve portion are arranged in the arc in a manner such that the elements adjacent to each other closely contact each other.
In the transmission belt described in the above-described publication, the rocking edge is formed at a predetermined distance d from the contact surface of the element (i.e., a so-called saddle surface), which contacts the innermost peripheral surface of the ring, as shown in FIG. 5. As described above, the elements adjacent to each other contact each other at the rocking edges in each belt curve portion. Accordingly, a radius of the transmission belt wound on the pulley, that is, a distance from a center of the pulley on which the transmission belt is wound to the innermost peripheral surface of the ring differs from a distance from the center of the pulley to the rocking edge. Therefore, a frictional force between the element and the ring differs from a frictional force between the element and the pulley. As a result, a relative slip may occur between the element and the ring.
If a relative slip occurs between the element and the ring, power transmission efficiency of the transmission belt formed by the elements and the ring may be reduced. Accordingly, to avoid a relative slip between the element and the ring, it is conceivable to make a position of the saddle surface coincide with a position of the rocking edge in the height direction of the element (i.e., in the direction of the radius of the transmission belt wounded on the pulley at the belt curve portion), as shown in FIG. 6. However, in an ordinary method of forming the conventional element, it is difficult to make the position of the saddle surface completely coincide with the position of the rocking edge in the height direction of the element. Further, there is a plate-thickness difference or a level difference between a portion in which the saddle surface is formed and a portion in which the rocking edge is formed.
The above-described conventional elements are generally produced in large quantities, mainly by a press-punching operation, using a metal plate material, in view of productivity and economic efficiency. However, in this case, in a shear surface of the formed element, a burr is inevitably formed by a shearing operation, and a shear droop is inevitably formed when the material is pressed down by a cutting blade before the shearing operation is started, as shown in FIG. 7. Therefore, if the rocking edge is formed on a side surface on which the shear droop is formed when the element is formed by the punching operation, the sharp rocking edge is formed only in the neck portion of the element, which is not sheared when the punching operation is performed, and in which the shear droop is not formed when the punching operation is performed. As a result, there is a plate-thickness difference Δt between the neck portion of the element, in which the substantive rocking edge is formed, and the portion in which the saddle surface is formed, as shown in FIG. 6.
If there is the plate-thickness difference, the elements adjacent to each other contact each other only at the rocking edges formed in the neck portions of the elements. As a result, a large bending moment is applied to the neck portion of the element, or stress concentration occurs due to the bending moment in the neck portion of the element. This reduces the durability of the element, that is, the durability of the transmission belt. When the rocking edge is formed on a side surface on which the burr is formed when the element is formed by the punching operation, the sharp rocking edge is formed to extend over an entire width of the element. However, in this case, there is a level difference between a corner portion, which is positioned between the side surface opposite to the side surface on which the rocking edge is formed and the saddle surface, and the neck portion of the element. As a result, a bending moment is applied to the neck portion of the element, or stress concentration occurs due to the bending moment in the neck portion of the element, as in the case where the rocking edge is formed on the surface on which the shear droop is formed.
Thus, improvement needs to be made to increase the power transmission efficiency of the transmission belt, while preventing a reduction of the durability of the transmission belt, using the elements formed by an ordinary press operation.